1. Field of the Invention
The present invention relates to an agitator tank that contains a magnetically coupled mixer.
2. Description of Related Art
Chemical compounds are typically mixed within an agitator tank that contains a rotating mixer. Agitator tanks are frequently used to mix ingredients for pharmaceuticals which require a very sterile environment during the mixing process. For example, the Food and Drug Administration (FDA) has recently promulgated strict sterile requirements on solutions being processed for intravenous use. The dangers of cell growth within the tank are of particularly concern. To provide a sterile environment, agitator tanks must be constructed to prevent contaminants from entering or remaining in the tanks during a batch process.
A typical agitator tank includes a drive shaft that couples a mixing impeller to a motor located outside of the tank. The mixers contain seals located between the drive shaft and the body of the tank. Seals are undesirable because such components are susceptible to failure, and are difficult to clean and replace. Additionally, the sealed mixers are typically located at the top of the tank. Overhead mixers are relatively inefficient in mixing, particularly ingredients located at the bottom of the tank.
To reduce the amount of sealing in an agitator tank, some tanks contain magnetic couplers which couple the mixing impeller to a motor located exterior to the tank. Magnetic couplers have a drive magnet attached to the motor and a driven magnet connected to the impeller. The magnets are assembled close together, so that rotation of the drive magnet rotates the driven magnet and impeller. The magnetic coupler allows the mixing impeller to be rotated by the motor without mechanically connecting the two members. Magnetic couplers can therefore be constructed so that there is no seal associated with the mixing impeller. Additionally, magnetic couplers typically require less horsepower than conventional overhead mixers.
Present magnetic couplers have a plurality of horizontally located magnets which couple the impeller to the motor. The magnetic force between the magnets of present couplers is relatively small, thereby limiting the size of the motor. For example, present magnetically coupled motors do not typically exceed 1.5 horsepower. The small magnetic forces can result in magnetic decoupling when the load on the impeller is greater than the torque of the magnets. The size of the tank and the viscosity of the contents is therefore limited to the size of the mixing motor. It would therefore be desirable to have a sealess mixer which can create a relatively large output torque. It would also be desirable to have a sealess mixer which was efficient in mixing the contents of an agitator tank.